Patent Publication Number: US-2022231540-A1

Title: Detection Of Device Removal From A Surface Of A Multi-Coil Wireless Charging Device

Description:
PRIORITY CLAIM 
     This application claims priority to and the benefit of non-provisional patent application Ser. No. 16/936,361 filed in the United States Patent Office on Jul. 22, 2020, provisional patent application No. 62/877,831 filed in the United States Patent Office on Jul. 23, 2019, of provisional patent application No. 63/019,241 filed in the United States Patent Office on May 1, 2020, of provisional patent application No. 63/019,245 filed in the United States Patent Office on May 1, 2020, and of provisional patent application No. 63/019,248 filed in the United States Patent Office on May 1, 2020, and the entire content of each of these applications is incorporated herein by reference as if fully set forth below in their entirety and for all applicable purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to wireless charging of batteries, including batteries in mobile computing devices, and more particularly to detection of device removal during a charging operation. 
     BACKGROUND 
     Wireless charging systems have been deployed to enable certain types of devices to charge internal batteries without the use of a physical charging connection. Devices that can take advantage of wireless charging include mobile processing and/or communication devices. Standards, such as the Qi standard defined by the Wireless Power Consortium enable devices manufactured by a first supplier to be wirelessly charged using a charger manufactured by a second supplier. Standards for wireless charging are optimized for relatively simple configurations of devices and tend to provide basic charging capabilities. 
     Improvements in wireless charging capabilities are required to support continually increasing complexity of mobile devices and changing form factors. For example, there is a need for a faster, lower power detection techniques that enable a charging device to detect and locate chargeable devices on a surface of a charging device, and to detect removal or relocation of a chargeable device during a wireless charging operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a charging cell that may be provided on a charging surface provided by a wireless charging device in accordance with certain aspects disclosed herein. 
         FIG. 2  illustrates an example of an arrangement of charging cells provided on a single layer of a segment of a charging surface provided by a wireless charging device in accordance with certain aspects disclosed herein. 
         FIG. 3  illustrates an example of an arrangement of charging cells when multiple layers of charging cells are overlaid within a segment of a charging surface provided by a wireless charging device in accordance with certain aspects disclosed herein. 
         FIG. 4  illustrates the arrangement of power transfer areas provided by a charging surface of a charging device that employs multiple layers of charging cells configured in accordance with certain aspects disclosed herein. 
         FIG. 5  illustrates a wireless transmitter that may be provided in a charger base station in accordance with certain aspects disclosed herein. 
         FIG. 6  illustrates a first example of a response to a passive ping in accordance with certain aspects disclosed herein. 
         FIG. 7  illustrates a second example of a response to a passive ping in accordance with certain aspects disclosed herein. 
         FIG. 8  illustrates examples of observed differences in responses to a passive ping in accordance with certain aspects disclosed herein. 
         FIG. 9  illustrates a first topology that supports matrix multiplexing switching for use in a wireless charger adapted in accordance with certain aspects disclosed herein. 
         FIG. 10  illustrates a second topology that supports direct current drive in a wireless charger adapted in accordance with certain aspects disclosed herein. 
         FIG. 11  illustrates a multi-coil wireless charging system configured to reliably detect removal of a receiving device in accordance with certain aspects of this disclosure. 
         FIG. 12  is a graphical representation of certain aspects of a device removal event that may be monitored in accordance with certain aspects disclosed herein. 
         FIG. 13  illustrates a filtered threshold detection circuit that employs low-pass filtering to accommodate variability in charging current or tank voltage in accordance with certain aspects disclosed herein. 
         FIG. 14  illustrates a Q-factor comparison circuit used for detection of removal of a receiving device in accordance with certain aspects disclosed herein. 
         FIG. 15  illustrates the use of look-up tables for detecting a device removal event in accordance with certain aspects disclosed herein. 
         FIG. 16  illustrates an example of a procedure that uses look-up tables when detecting a device removal event in accordance with certain aspects disclosed herein. 
         FIG. 17  illustrates the use of measured quiescent or idle transfer power draw for detecting a device removal event in accordance with certain aspects disclosed herein. 
         FIG. 18  illustrates a first example of a procedure for device removal detection based on measured quiescent power draw according to certain aspects disclosed herein. 
         FIG. 19  illustrates a second example of a procedure for device removal detection based on measured quiescent power draw according to certain aspects disclosed herein. 
         FIG. 20  illustrates a first example of the use of a measurement slot to perform a ping procedure according to certain aspects disclosed herein. 
         FIG. 21  illustrates a second example of the use of a measurement slot to perform a ping procedure according to certain aspects disclosed herein. 
         FIG. 22  illustrates a first example of the use of sensors to detect removal of a receiving device during power transfer in accordance with certain aspects disclosed herein. 
         FIG. 23  illustrates a second example of the use of sensors to detect removal of a receiving device during power transfer in accordance with certain aspects disclosed herein. 
         FIG. 24  illustrates a third example of the use of sensors to detect removal of a receiving device during power transfer in accordance with certain aspects disclosed herein. 
         FIG. 25  illustrates a fourth example of the use of sensors to detect removal of a receiving device during power transfer in accordance with certain aspects disclosed herein. 
         FIG. 26  illustrates a fifth example of the use of sensors to detect removal of a receiving device during power transfer in accordance with certain aspects disclosed herein. 
         FIG. 27  illustrates one example of an apparatus employing a processing circuit that may be adapted according to certain aspects disclosed herein. 
         FIG. 28  illustrates a method for operating a charging device in accordance with certain aspects of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     Several aspects of wireless charging systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawing by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as “elements”). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a “processing system” that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a processor-readable storage medium. A processor-readable storage medium, which may also be referred to herein as a computer-readable medium may include, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., compact disk (CD), digital versatile disk (DVD)), a smart card, a flash memory device (e.g., card, stick, key drive), Near Field Communications (NFC) token, random access memory (RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), a register, a removable disk, a carrier wave, a transmission line, and any other suitable medium for storing or transmitting software. The computer-readable medium may be resident in the processing system, external to the processing system, or distributed across multiple entities including the processing system. Computer-readable medium may be embodied in a computer-program product. By way of example, a computer-program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     Overview 
     Certain aspects of the present disclosure relate to systems, apparatus and methods applicable to wireless charging devices and techniques. Charging cells may be configured with one or more inductive coils to provide a charging surface in a charging device where the charging surface enables the charging device to charge one or more chargeable devices wirelessly. The location of a device to be charged may be detected through sensing techniques that associate location of the device to changes in a physical characteristic centered at a known location on the charging surface. Sensing of location may be implemented using capacitive, resistive, inductive, touch, pressure, load, strain, and/or another appropriate type of sensing. 
     In one aspect of the disclosure, an apparatus has a battery charging power source, a plurality of charging cells configured in a matrix, a first plurality of switches in which each switch is configured to couple a row of coils in the matrix to a first terminal of the battery charging power source, and a second plurality of switches in which each switch is configured to couple a column of coils in the matrix to a second terminal of the battery charging power source. Each charging cell in the plurality of charging cells may include one or more coils surrounding a power transfer area. The plurality of charging cells may be arranged adjacent to the charging surface of the charging device without overlap of power transfer areas of the charging cells in the plurality of charging cells. 
     In some instances, the apparatus may also be referred to as a charging surface. Power can be wirelessly transferred to a receiving device located anywhere on a surface of the apparatus. The devices can have an arbitrarily defined size and/or shape and may be placed without regard to any discrete placement locations enabled for charging. Multiple devices can be simultaneously charged on a single charging surface. The apparatus can track motion of one or more devices across the charging surface. 
     Charging Cells 
     According to certain aspects disclosed herein, a charging surface may be provided using charging cells in a charging device, where the charging cells are deployed adjacent to the charging surface. In one example the charging cells are deployed in one or more layers of the charging surface in accordance with a honeycomb packaging configuration. A charging cell may be implemented using one or more coils that can each induce a magnetic field along an axis that is substantially orthogonal to the charging surface adjacent to the coil. In this description, a charging cell may refer to an element having one or more coils where each coil is configured to produce an electromagnetic field that is additive with respect to the fields produced by other coils in the charging cell and directed along or proximate to a common axis. 
     In some implementations, a charging cell includes coils that are stacked along a common axis and/or that overlap such that they contribute to an induced magnetic field substantially orthogonal to the charging surface. In some implementations, a charging cell includes coils that are arranged within a defined portion of the charging surface and that contribute to an induced magnetic field within the substantially orthogonal portion of the charging surface associated with the charging cell. In some implementations, charging cells may be configurable by providing an activating current to coils that are included in a dynamically-defined charging cell. For example, a charging device may include multiple stacks of coils deployed across the charging surface, and the charging device may detect the location of a device to be charged and may select some combination of stacks of coils to provide a charging cell adjacent to the device to be charged. In some instances, a charging cell may include, or be characterized as a single coil. However, it should be appreciated that a charging cell may include multiple stacked coils and/or multiple adjacent coils or stacks of coils. The coils may be referred to herein as charging coils, wireless charging coils, transmitter coils, transmitting coils, power transmitting coils, power transmitter coils, or the like. 
       FIG. 1  illustrates an example of a charging cell  100  that may be deployed and/or configured to provide a charging surface of a charging device. As described herein, the charging surface may include an array of charging cells  100  provided on one or more substrates  106 . A circuit comprising one or more integrated circuits (ICs) and/or discrete electronic components may be provided on one or more of the substrates  106 . The circuit may include drivers and switches used to control currents provided to coils used to transmit power to a receiving device. The circuit may be configured as a processing circuit that includes one or more processors and/or one or more controllers that can be configured to perform certain functions disclosed herein. In some instances, some or all of the processing circuit may be provided external to the charging device. In some instances, a power supply may be coupled to the charging device. 
     The charging cell  100  may be provided in close proximity to an outer surface area of the charging device, upon which one or more devices can be placed for charging. The charging device may include multiple instances of the charging cell  100 . In one example, the charging cell  100  has a substantially hexagonal shape that encloses one or more coils  102 , which may be constructed using conductors, wires or circuit board traces that can receive a current sufficient to produce an electromagnetic field in a power transfer area  104 . In various implementations, some coils  102  may have a shape that is substantially polygonal, including the hexagonal charging cell  100  illustrated in  FIG. 1 . Other implementations provide coils  102  that have other shapes. The shape of the coils  102  may be determined at least in part by the capabilities or limitations of fabrication technology, and/or to optimize layout of the charging cells on a substrate  106  such as a printed circuit board substrate. Each coil  102  may be implemented using wires, printed circuit board traces and/or other connectors in a spiral configuration. Each charging cell  100  may span two or more layers separated by an insulator or substrate  106  such that coils  102  in different layers are centered around a common axis  108 . 
       FIG. 2  illustrates an example of an arrangement  200  of charging cells  202  provided on a single layer of a segment of a charging surface of a charging device that may be adapted in accordance with certain aspects disclosed herein. The charging cells  202  are arranged according to a honeycomb packaging configuration. In this example, the charging cells  202  are arranged end-to-end without overlap. This arrangement can be provided without through-hole or wire interconnects. Other arrangements are possible, including arrangements in which some portion of the charging cells  202  overlap. For example, wires of two or more coils may be interleaved to some extent. 
       FIG. 3  illustrates an example of an arrangement of charging cells from two perspectives  300 ,  310  when multiple layers are overlaid within a segment of a charging surface that may be adapted in accordance with certain aspects disclosed herein. Layers of charging cells  302 ,  304 ,  306 ,  308  are provided within a segment of a charging surface. The charging cells within each layer of charging cells  302 ,  304 ,  306 ,  308  are arranged according to a honeycomb packaging configuration. In one example, the layers of charging cells  302 ,  304 ,  306 ,  308  may be formed on a printed circuit board that has four or more layers. The arrangement of charging cells  100  can be selected to provide complete coverage of a designated charging area that is adjacent to the illustrated segment. 
       FIG. 4  illustrates the arrangement of power transfer areas provided in a charging surface  400  that employs multiple layers of charging cells configured in accordance with certain aspects disclosed herein. The illustrated charging surface is constructed from four layers of charging cells  402 ,  404 ,  406 ,  408 . In  FIG. 4 , each power transfer area provided by a charging cell in the first layer of charging cells  402  is marked “L1”, each power transfer area provided by a charging cell in the second layer of charging cells  404  is marked “L2”, each power transfer area provided by a charging cell in the third layer of charging cells  406  is marked “L3”, and each power transfer area provided by a charging cell in the fourth layer of charging cells  408  is marked “L4”. 
     Wireless Transmitter 
       FIG. 5  illustrates a wireless transmitter  500  that may be provided in a charger base station. A controller  502  may receive a feedback signal filtered or otherwise processed by a conditioning circuit  508 . The controller may control the operation of a driver circuit  504  that provides an alternating current to a resonant circuit  506  that includes a capacitor  512  and inductor  514 . The resonant circuit  506  may also be referred to herein as a tank circuit, LC tank circuit, or LC tank, and the voltage  516  measured at an LC node  510  of the resonant circuit  506  may be referred to as the tank voltage. 
     The wireless transmitter  500  may be used by a charging device to determine if a compatible device has been placed on a charging surface. For example, the charging device may determine that a compatible device has been placed on the charging surface by sending an intermittent test signal (active ping) through the wireless transmitter  500 , where the resonant circuit  506  may detect or receive encoded signals when a compatible device responds to the test signal. The charging device may be configured to activate one or more coils in at least one charging cell after receiving a response signal defined by standard, convention, manufacturer or application. In some examples, the compatible device can respond to a ping by communicating received signal strength such that the charging device can find an optimal charging cell to be used for charging the compatible device. 
     Passive ping techniques may use the voltage and/or current measured or observed at the LC node  510  to identify the presence of a receiving coil in proximity to the charging pad of a device adapted in accordance with certain aspects disclosed herein. In many conventional wireless charger transmitters, circuits are provided to measure voltage at the LC node  510  or to measure the current in the LC network. These voltages and currents may be monitored for power regulation purposes or to support communication between devices. In the example illustrated in  FIG. 5 , voltage at the LC node  510  is monitored, although it is contemplated that current may additionally or alternatively be monitored to support passive ping in which a short pulse is provided to the resonant circuit  506 . A response of the resonant circuit  506  to a passive ping (initial voltage V 0 ) may be represented by the voltage (V LC ) at the LC node  510 , such that: 
     
       
         
           
             
               
                 
                   
                     
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     According to certain aspects disclosed herein, coils in one or more charging cells may be selectively activated to provide an optimal electromagnetic field for charging a compatible device. In some instances, coils may be assigned to charging cells, and some charging cells may overlap other charging cells. In the latter instances, the optimal charging configuration may be selected at the charging cell level. In other instances, charging cells may be defined based on placement of a device to be charged on a surface of the charging device. In these other instances, the combination of coils activated for each charging event can vary. In some implementations, a charging device may include a driver circuit that can select one or more cells and/or one or more predefined charging cells for activation during a charging event. 
       FIG. 6  illustrates a first example in which a response  600  to a passive ping decays according to Equation 3. After the excitation pulse at time t=0, the voltage and/or current is seen to oscillate at the resonant frequency defined by Equation 1, and with a decay rate defined by Equation 3. The first cycle of oscillation begins at voltage level V 0  and V LC  continues to decay to zero as controlled by the Q factor and ω. The example illustrated in  FIG. 6  represents a typical open or unloaded response when no object is present or proximate to the charging pad. In  FIG. 6  the value of the Q factor is assumed to be  20 . 
       FIG. 7  illustrates a second example in which a response  700  to a passive ping decays according to Equation 3. After the excitation pulse at time=0, the voltage and/or current is seen to oscillate at the resonant frequency defined by Equation 1, and with a decay rate defined by Equation 3. The first cycle of oscillation begins at voltage level V 0  and V LC  continues to decay to zero as controlled by the Q factor and ω. The example illustrated in  FIG. 7  represents a loaded response when an object is present or proximate to the charging pad loads the coil. In  FIG. 7  the Q factor may have a value of  7 . V LC  oscillates at a higher in the response  700  with respect to the response  600 . 
       FIG. 8  illustrates a set of examples in which differences in responses  800 ,  820 ,  840  may be observed. A passive ping is initiated when a driver circuit  504  excites the resonant circuit  506  using a pulse that is shorter than 2.5 μs. Different types of wireless receivers and foreign objects placed on the transmitter result in different responses observable in the voltage at the LC node  510  or current in the resonant circuit  506  of the transmitter. The differences may indicate variations in the Q factor of the resonant circuit  506  frequency of the oscillation of V 0 . Table 1 illustrates certain examples of objects placed on the charging pad in relation to an open state. 
                                     TABLE 1                       V peak     50% Decay   Q       Object   Frequency   (mV)   Cycles   Factor                                                    None present   96.98 kHz   134 mV   4.5   20.385       Type-1 Receiver   64.39 kHz    82 mV   3.5   15.855       Type-2 Receiver   78.14 kHz    78 mV   3.5   15.855       Type-3 Receiver   76.38 kHz   122 mV   3.2   14.496       Misaligned Type-3   210.40 kHz    110 mV   2.0   9.060       Receiver       Ferrous object   93.80 kHz   110 mV   2.0   9.060       Non-ferrous object   100.30 kHz    102 mV   1.5   6.795                    
In Table 1, the Q factor may be calculated as follows:
 
     
       
         
           
             
               
                 
                   
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     where N is the number of cycles from excitation until amplitude falls below 0.5 V 0 . 
     Selectively Activating Coils 
     According to certain aspects disclosed herein, transmitting coils in one or more charging cells may be selectively activated to provide an optimal electromagnetic field for charging a compatible device. In some instances, transmitting coils may be assigned to charging cells, and some charging cells may overlap other charging cells. In the latter instances, the optimal charging configuration may be selected at the charging cell level. In other instances, charging cells may be defined based on placement of a device to be charged on a charging surface. In these other instances, the combination of coils activated for each charging event can vary. In some implementations, a charging device may include a driver circuit that can select one or more cells and/or one or more predefined charging cells for activation during a charging event. 
       FIG. 9  illustrates a first topology  900  that supports matrix multiplexing switching for use in a wireless charger adapted in accordance with certain aspects disclosed herein. The wireless charger may select one or more charging cells  100  to charge a receiving device. Charging cells  100  that are not in use can be disconnected from current flow. A relatively large number of charging cells  100  may be used in the honeycomb packaging configuration illustrated in  FIG. 2  requiring a corresponding number of switches. According to certain aspects disclosed herein, the charging cells  100  may be logically arranged in a matrix  908  having multiple cells connected to two or more switches that enable specific cells to be powered. In the illustrated topology  900 , a two-dimensional matrix  908  is provided, where the dimensions may be represented by X and Y coordinates. Each of a first set of switches  906  is configured to selectively couple a first terminal of each cell in a column of cells to a wireless transmitter and/or receiver circuit  902  that provide current to activate coils during wireless charging. Each of a second set of switches  904  is configured to selectively couple a second terminal of each cell in a row of cells to the wireless transmitter and/or receiver circuit  902 . A cell is active when both terminals of the cell are coupled to the wireless transmitter and/or receiver circuit  902 . 
     The use of a matrix  908  can significantly reduce the number of switching components needed to operate a network of tuned LC circuits. For example, N individually connected cells require at least N switches, whereas a two-dimensional matrix  908  having N cells can be operated with AIN switches. The use of a matrix  908  can produce significant cost savings and reduce circuit and/or layout complexity. In one example, a 9-cell implementation can be implemented in a 3×3 matrix  908  using 6 switches, saving 3 switches. In another example, a 16-cell implementation can be implemented in a 4×4 matrix  908  using 8 switches, saving 8 switches. 
     During operation at least 2 switches are closed to actively couple one coil to a wireless transmitter and/or receiver circuit  902 . Multiple switches can be closed at once in order to facilitate connection of multiple coils to the wireless transmitter and/or receiver circuit  902 . Multiple switches may be closed, for example, to enable modes of operation that drive multiple transmitting coils when transferring power to a receiving device. 
       FIG. 10  illustrates a second topology  1000  in which each coil or charging cell is individually and/or directly driven by a driver circuit  1002  in accordance with certain aspects disclosed herein. The driver circuit  1002  may be configured to select one or more coils or charging cells  100  from a group of coils  1004  to charge a receiving device. It will be appreciated that the concepts disclosed here in relation to charging cells  100  may be applied to selective activation of individual coils or stacks of coils. Charging cells  100  that are not in use receive no current flow. A relatively large number of charging cells  100  may be in use and a switching matrix may be employed to drive individual coils or groups of coils. In one example, a first switching matrix may configure connections that define a charging cell or a group of coils to be used during a charging event and a second switching matrix (see, e.g.,  FIG. 9 ) may be used to activate the charging cell and/or a group of selected coils. 
     Detecting Device Removal From A Multi-Coil Wireless Charger 
     With reference now to  FIG. 11 , a multi-coil wireless charging system  1100  provided in accordance with certain aspects of this disclosure can be configured to reliably detect removal of a receiving device  1106  while charging is in progress. Arbitrary and/or unanticipated removal of the receiving device can cause damage to other receiving devices  1108 , in addition to potential loss of detection efficiency for an approaching device  1108 . The multi-coil wireless charging system  1100  provides a charging surface  1102  that includes multiple transmitting coils  1104   1 - 1104   n . In the illustrated example, a receiving device  1106  is removed while receiving a charging flux from the n th  transmitting coil (transmitting coil  1104   n ). 
     In some instances, the charging surface  1102  continues to provide a charging current to the transmitting coil  1104   n  after the receiving device  1106  has been removed. The approaching device  1108  may be placed on the charging surface  1102  while the charging current is flowing. The charging current is typically configured based on the capabilities of the receiving device  1106 , which may differ from the capabilities of the approaching device  1108 . Damage to the approaching device  1108  can occur if the approaching device  1108  is not designed to handle the level of induced current intended for the original receiving device  1106 . 
     Certain aspects of this disclosure enable the multi-coil wireless charging system  1100  to rapidly and reliably detect the removal of the receiving device  1106  from the charging surface  1102 . The multi-coil wireless charging system  1100  may discontinue the flow of the charging current to the active transmitting coil  1104   n  upon detecting the removal of the receiving device  1106 . The multi-coil wireless charging system  1100  may configure the charging surface  1102  to detect objects, including the approaching device  1108  upon detecting the removal of the receiving device  1106  and discontinuation of the charging current. 
     According to certain aspects of the disclosure, removal of the receiving device  1106  may be detected by monitoring charging circuits, or certain characteristics of one or more of the transmitting coils  1104   1 - 1104   n . In certain examples, the removal of the receiving device  1106  may be detected based on changes in measured electrical quantities that can be attributed to changes in electromagnetic coupling between the transmitting coil  1104   n  and a receiving coil in the receiving device  1106 . 
     In one example, Dynamic Inferred Coupling Estimation (DICE) may be used to detect quality of coupling in real-time. DICE may include an evaluation of the ratio of real power to reactive power in a circuit that includes a transmitting coil and series resonant capacitor. The amount of reactive power stored in the inductor-capacitor (LC) circuit of the transmitter is substantially influenced by the coupling coefficient. The coupling coefficient defines the ratio of mutual inductance to leakage inductance in the LC circuit of the wireless transmitter. For example, leakage inductance in the LC circuit of the wireless transmitter may be expressed as: 
         Tx   leakage   =L   Tx ×(1− k ),   (Eq. 3)
 
     where L Tx  represents the self-inductance of the transmitter coil, and k represents the coupling coefficient. Decreasing coupling reduces coupling coefficient and increases leakage inductance, resulting in more reactive energy being stored in the leakage inductance of the transmitter. Energy stored in the leakage inductance does not contribute to power transfer and, as energy builds up in the leakage inductance, the voltage at the LC node increases. 
     Certain aspects of the coupling between one or more transmitting coils  1104   1 - 1104   n  and a receiving device  1106  may be characterized by voltage measured at the LC node. Voltage measurements taken at the LC node may be available for other reasons. In some instances, voltage at the LC node may be monitored as an overvoltage indicator used to protect power electronics and the resonant capacitor. In one example, the measurement circuit includes a voltage comparator configured to detect voltages exceeding a threshold level. According to certain aspects disclosed herein, a measurement circuit may be added, or an existing measurement circuit may be used to quantify or compare a voltage at the LC node that varies directly with the quality of coupling. 
       FIG. 12  is a graphical representation  1200  of certain aspects of a device removal event that may be monitored in accordance with certain aspects disclosed herein. Two curves  1202 ,  1204  represent the state of electrical quantities that are measurable in the multi-coil wireless charging system  1100 . 
     A first curve  1202  represents the magnitude of the current flowing to one or more active transmitting coils  1104   1 - 1104   n  to charge a receiving device  1106 . The receiving device  1106  is initially placed in proximity to the charging surface  1102  and is wirelessly receiving power. The receiving device  1106  then begins to move away from the charging surface  1102 , commencing at a first point in time  1206  (t 1 ), until the receiving device  1106  is receiving no power or an insignificant level of power from the active transmitting coils  1104   1 - 1104   n  at a second point in time  1208  (t 2 ). As illustrated in  FIG. 12 , the charging current can be expected to drop off when the receiving device  1106  is removed. The first curve  1202  includes a step between the initial level of the charging current and the level of the charging current or quiescent current after the receiving device  1106  has been removed. A sharp drop-off in charging current may be observed, even when the receiving device  1106  is removed at moderate rate, given the inverse-square relationship associated with electromagnetic coupling when the distance between transmitter and receiver is increasing. 
     A second curve  1204  represents magnitude of the tank voltage measured at an LC node in a resonant circuit that includes one or more active transmitting coils  1104   1 - 1104   n  used to wirelessly charge a receiving device  1106 . The receiving device  1106  that is initially placed in proximity to the charging surface  1102  and is wirelessly receiving power begins to be move away from the charging surface  1102 , commencing at the first point in time  1206  (t 1 ), and continues moving away until the receiving device  1106  is receiving no power or an insignificant power from the active transmitting coils  1104   1 - 1104   n  at the second point in time  1208  (t 2 ). As illustrated in  FIG. 12 , the tank voltage can be expected to increase with the impedance of the resonant circuit that results from the receiving device  1106  being removed. The second curve  1204  includes a step between the initial level of the tank voltage and the level of the tank voltage after the receiving device  1106  has been removed. A sharp increase in impedance and tank voltage may be observed, even when the receiving device  1106  is removed at moderate rate, given the inverse-square relationship associated with electromagnetic coupling when the distance between transmitter and receiver is increasing. 
     In accordance with certain aspects of this disclosure, charging current provided to the one or more active transmitting coils  1104   1 - 1104   n  and/or the tank voltage may be monitored during power transfers. The charging current may be discontinued when a step in current or voltage exceeds a threshold difference value or when the rate of change in current (di/dt) or voltage (dv/dt) exceeds a threshold rate of change. The threshold difference value and/or the threshold rate of change may be preconfigured by application, during system initialization and/or during manufacture or assembly. In some implementations the threshold difference value and/or the threshold rate of change may be dynamically configured based on a charging configuration identifying a number of transmitting coils  1104   1 - 1104 n to be used for wireless, charging the size of the charging current, and/or the structure or internal configuration of the receiving device  1106 . 
     In some instances, variability in charging current or tank voltage may be observed when the receiving device  1106  is not being removed. For example, charging current or tank voltage may vary due to vibration or slippage of the receiving device  1106  or charging surface  1102 , physical instability caused by motion of the multi-coil wireless charging system  1100 , or due to variations in temperature or drift in power supply output. Certain implementations may employ low-pass filtering to accommodate such variability in charging current or tank voltage. 
       FIG. 13  illustrates a filtered threshold detection circuit  1300  that employs low-pass filtering to accommodate variability in charging current or tank voltage that is not attributable to movement of the receiving device  1106 .  FIG. 13  includes a graph  1320  that illustrates certain aspects related to the operation of the filtered threshold detection circuit  1300 . In the illustrated example, the filtered threshold detection circuit  1300  receives an input signal  1310  representative of the charging current flowing to one or more active transmitting coils  1104   1 - 1104   n  used to charge the receiving device  1106 . The input signal  1310  is provided to a low-pass filter  1302  that can delay step changes in the input signal  1310 , and/or that slows the rate of change in the input signal  1310 . A comparison circuit  1304  compares the output  1312  of the low-pass filter  1302  with a threshold signal  1314  generated from the input signal  1310  using a scaling factor  1308  or offset that is based on the prior state of the output  1316  of the comparison circuit  1304 . The threshold signal  1314  may be generated by a feedback circuit  1306  to provide hysteresis in the filtered threshold detection circuit  1300 . The threshold signal  1314  provides a reference point  1330  that enables the comparison circuit  1304  to reliably indicate removal of the receiving device  1106 . The low-pass filter  1302  may be configured with a filter constant configured such that normal small variations in current  1332  do not cause a device removal indication. 
     A first curve  1322  represents the magnitude of a current flowing to one or more active transmitting coils  1104   1 - 1104   n  to charge a receiving device  1106 . The receiving device  1106  is initially placed in proximity to the charging surface  1102  and is wirelessly receiving power. A second curve  1324  represents the threshold value used to determine when a step change in charging current indicates a device removal event. The receiving device  1106  begins to move away from the charging surface  1102 , commencing at a first point in time  1326  (t 1 ), until the receiving device  1106  is receiving no power or an insignificant power from the active transmitting coils  1104   1 - 1104   n , at a second point in time  1328  (t 2 ). 
     The charging current drops off when the receiving device  1106  is removed resulting in a step between the initial level of the charging current and the level of the charging current or quiescent current after the receiving device  1106  has been removed. The threshold signal  1314  can ensure that large step changes in the charging current (or large increases in tank voltage) is sufficient to cross the threshold value. 
     In another aspect of the disclosure, slot-based techniques may be used to enable detection of removal of a receiving device  1106 . In one example, a time-slot is provided during which charging current is suspended a short period of time to enable one or more measurements and/or the interrogation of one or more sensors. 
       FIG. 14  illustrates a Q-factor comparison circuit  1400  and corresponding timing diagram  1420  that illustrate detection of removal of a receiving device  1106  during a measurement slot  1424 . The timing diagram  1420  includes a curve  1422  representing magnitude of a charging current flowing in one or more active transmitting coils  1104   1 - 1104   n  when a charging surface  1102  of a multi-coil wireless charging system  1100  is configured to charge a receiving device  1106 . The measurement slot  1424  may be provided periodically or in response to detection of a step change in the magnitude of the charging current or tank voltage. A slotted Q-factor test may be performed during the measurement slot  1424 . The measurement slot  1424  may be provided when the multi-coil wireless charging system  1100  suspends or terminates the charging current. In one example, the measurement slot  1424  has a duration of up to 100 microseconds (μs). Energy stored in the resonant circuit decays at rate that is determined in part by the Q-factor of the resonant circuit. The Q-factor of the resonant circuit may be used as a measure of the electromagnetic coupling between certain active transmitting coils  1104   1 - 1104   n  in the charging surface  1102  and the receiving coil in the receiving device  1106 . 
     A slotted Q-factor test may commence at a first point in time  1426  (t 1 ), when the multi-coil wireless charging system  1100  ceases to power the resonant circuit that includes the active transmitting coils  1104   1 - 1104   n . The magnitude of the current in the resonant circuit decays  1428  at a rate determined by the Q-factor of the resonant circuit. In one example, a Q-factor  1402  may be calculated and compared to a reference Q-factor  1404  using a comparator  1406 . The reference Q-factor  1404  may correspond to a Q-factor calculated when no receiving device is electromagnetically coupled to the active transmitting coils  1104   1 - 1104   n . 
     In some implementations, the filtered threshold detection circuit  1300  may be used to compare calculated Q-factor with a threshold Q-factor. In some implementations, a measurement slot  1424  is provided periodically to enable the slotted Q-factor test to be used to detect presence of foreign objects as well as device removal events. 
     Another aspect of this disclosure relates to detecting the removal of a receiving device  1106  using threshold values and other parameters that are maintained in lookup tables. For example, lookup tables may be used to maintain measured values of charging current, tank voltage, Q-factor and other characteristics of a multi-coil wireless charging system  1100 . In some implementations, the lookup tables may maintain threshold values and other parameters for different charging configurations. Each charging configuration may define a set of transmitting coils  1104   1 - 1104   n  to be used for charging the receiving device  1106  and a current distribution among the transmitting coils  1104   1 - 1104   n . For example, one or more charging configurations may define phase offsets for currents provided to different transmitting coils  1104   1 - 1104   n  when an electromagnetic flux is to be concentrated at a specific location or directed within an area spanned by the transmitting coils  1104   1 - 1104   n . One or more charging configurations may be provided to match the capabilities, location, orientation, charging state, and/or another characteristic of the receiving device  1106 . The use of lookup tables can improve the efficiency of detection circuits and processes used to determine when a receiving device  1106  has been removed. 
       FIG. 15  includes graphs  1500 ,  1520  illustrating the use of look-up tables for detecting a device removal event that may be monitored in accordance with certain aspects disclosed herein. In one example, a lookup table (LUT) may maintain information identifying a known quiescent or “empty” power and/or a known current draw for a charging configuration. The multi-coil wireless charging system  1100  may compare measured power, voltage and/or current after a step event has been detected with a threshold value or other corresponding value for quiescent power, voltage and/or current draw maintained in a lookup table. The comparison may indicate whether the step event corresponds to a load change or a device removal. A load change may occur when a receiving device dumps its load. The lookup table may include values for quiescent power, voltage and/or current measured for one or more transmitting coils  1104   1 - 1104   n  that are not electromagnetically coupled to a receiving coil or another object that may affect the resonant frequency or Q-factor of the resonant circuit that includes the more transmitting coils  1104   1 - 1104   n . In some implementations, the lookup table may be populated with values measured for different charging configurations. In some implementations, the lookup table may be populated during a system configuration or calibration procedure. 
     The first graph  1500  illustrates an example in which the thresholds  1506 ,  1508  identified in a lookup table can reliably indicate that the receiving device  1106  has been removed. The second graph  1520  illustrates an example in which the thresholds  1526 ,  1528  maintained in a lookup table can reliably indicate that a load change has occurred. A first curve  1502 ,  1522  represents the magnitude of the current flowing to one or more active transmitting coils  1104   1 - 1104   n  to charge a receiving device  1106 . The receiving device  1106  is initially placed in proximity to the charging surface  1102  and is wirelessly receiving power. The receiving device  1106  then begins to move away from the charging surface  1102 , commencing at a first point in time  1512 ,  1532  (t 1 ), until the receiving device  1106  is receiving reduced power, corresponding to a drop-off in the charging current. Each of the first curves  1502 ,  1522  includes a step  1510 ,  1530  between the initial level of the charging current and the level of the charging current after the receiving device  1106  has been removed. 
     In one aspect of the disclosure, the magnitude of the charging current measured after the step  1510 ,  1530  is compared to a current threshold  1508 ,  1528  (or reference quiescent current level) obtained from a lookup table. In one example, the multi-coil wireless charging system  1100  may terminate the charging current based on the difference between the charging current level after the step  1510 ,  1530  and a reference quiescent current level or current threshold  1508 ,  1528 . In the example illustrated by the first graph  1500 , the multi-coil wireless charging system  1100  may terminate the charging current when the charging current level is within a configured range that includes a reference quiescent current level or is less than a current threshold  1508 ,  1528  calculated using the reference quiescent current level. In the example illustrated by the second graph  1520 , the multi-coil wireless charging system  1100  may continue to provide the charging current when the charging current level is greater than the current threshold  1508 ,  1528  by an amount that indicates a load change event has occurred. 
     A second curve  1504 ,  1524  in the graphs  1500 ,  1520  represents the magnitude of the tank voltage measured across a resonant circuit that includes to one or more active transmitting coils  1104   1 - 1104   n . The receiving device  1106  is initially placed in proximity to the charging surface  1102  and is wirelessly receiving power. The receiving device  1106  then begins to move away from the charging surface  1102 , commencing at a first point in time  1512 ,  1532  (t 1 ), until the receiving device  1106  is receiving reduced power, corresponding to an increase in the tank voltage. Each of the second curves  1504 ,  1524  includes a step  1510 ,  1530  between the initial level of the tank voltage and the level of the tank voltage after the receiving device  1106  has been removed. 
     In one aspect of the disclosure, the magnitude of the tank voltage measured after the step  1510 ,  1530  is compared to a reference quiescent tank voltage or a voltage threshold  1506 ,  1526  obtained from a lookup table. The multi-coil wireless charging system  1100  may terminate the charging current based on the difference between the tank voltage level after the step  1510 ,  1530  and the reference quiescent tank voltage or the voltage threshold  1506 ,  1526 . In the example illustrated by the first graph  1500 , the multi-coil wireless charging system  1100  may terminate the charging current when the tank voltage level is within a configured range that includes the quiescent tank voltage or is greater than the voltage threshold  1506 ,  1526 . In the example illustrated by the second graph  1520 , the multi-coil wireless charging system  1100  may continue to provide the charging current when the tank voltage is less than the voltage threshold  1506 ,  1526  indicating that a load change event has occurred. 
       FIG. 16  is a flowchart  1600  that illustrates an example of a procedure based on the examples illustrated in  FIG. 15 . The procedure may be performed at a multi-coil wireless charging system  1100 . At block  1602 , the multi-coil wireless charging system  1100  may begin providing a charging current to a receiving device  1106  in accordance with a charging configuration. The multi-coil wireless charging system  1100  may continue charging until at block  1604 , the multi-coil wireless charging system  1100  detects a step change in a measured value. In one example, the measured value may represent the magnitude of the charging current. In another example, the measured value may represent a tank voltage. At block  1606 , the multi-coil wireless charging system  1100  may measure the value after the step. At block  1608 , the multi-coil wireless charging system  1100  may compare the measured value to a threshold stored in a lookup table. The threshold may be calculated from an idle or quiescent value. The relationship between the measured value and the threshold may indicate whether the step change in the measured value is the result of removal of the receiving device  1106 . When the multi-coil wireless charging system  1100  determines at block  1610  that the step change relates to a device removal event, then at block  1612 , the multi-coil wireless charging system  1100  may terminate the charging current. If the multi-coil wireless charging system  1100  determines at block  1610  that the step change does not relate a device removal event, then the process may continue at block  1604 . 
       FIG. 17  is a graph  1700  that illustrates the use of measured quiescent power draw or a preconfigured, or premeasured idle transfer power draw value maintained in a look-up table for detecting a device removal event in accordance with certain aspects disclosed herein. In one aspect, a measurement of power transfer is obtained during an initial configuration interval period  1702  that may be associated with a ping procedure. The curve  1710  represents power or current transfer from the multi-coil wireless charging system  1100  to the receiving device  1106 . In one example, a measured power transfer value that characterizes a minimal or quiescent power transfer state may be used to set a known operating point for the multi-coil wireless charging system  1100 . The known operating point may be used to define a threshold for detecting device removal. The latter threshold may be referred to as the Measured Threshold  1716  herein. In another aspect, a threshold for detecting device removal may be obtained from a lookup table. The latter threshold may be referred to as the LUT Threshold  1718  herein. The LUT Threshold  1718  may be calculated or measured during system initialization, assembly or during a calibration procedure. In one example, the LUT Threshold  1718  may be calculated or measured when no chargeable device or other object is located on or near the charging surface  1102 . 
     The curve  1710  may correspond to a charging current that enables power to be transferred from the multi-coil wireless charging system  1100  to the receiving device  1106 . After initial detection and/or configuration of the receiving device  1106 , a minimal power transfer level  1712  may be determined and/or used to set the Measured Threshold  1716 . A power transfer period  1704  ensues. The power transfer period  1704  continues until an event  1706  is detected, where the level of power transfer exhibits a step drop. In the illustrated example, the level of power transfer drops to a lower level  1714  that may be above or below the threshold used to determine device removal. The threshold may be selected from Measured Threshold  1716  or LUT Threshold  1718 . The multi-coil wireless charging system  1100  may initiate a measurement slot  1708  in order to establish or confirm a device removal has occurred. During the measurement slot  1708 , the multi-coil wireless charging system  1100  may measure and compare quiescent power draw to the selected threshold. The multi-coil wireless charging system  1100  may discontinue the charging current after determining that the receiving device  1106  has been removed. The multi-coil wireless charging system  1100  may continue the power transfer at the lower level  1714  after determining that the receiving device  1106  has not been removed. 
       FIG. 18  is a flowchart  1800  that illustrates a first example of a procedure for device removal detection based on measured quiescent power draw. The procedure may be performed at a multi-coil wireless charging system  1100 . At block  1802 , the multi-coil wireless charging system  1100  may detect the presence of a receiving device  1106  that has been placed on or near the charging surface  1102 . During an initial configuration interval period  1702 , the multi-coil wireless charging system  1100  may interrogate and/or negotiate with the receiving device  1106  to generate a charging configuration. At block  1804 , the multi-coil wireless charging system  1100  may provide a quiescent current to one or more active transmitting coils  1104   1 - 1104   n  and may measure quiescent power draw. The multi-coil wireless charging system  1100  may use the measured quiescent power draw to establish a Measured Threshold  1716 . In one example, the Measured Threshold  1716  may be stored in non-volatile memory such as random-access memory (RAM) or register-based memory. 
     The power transfer period  1704  begins, during which the multi-coil wireless charging system  1100  may provide a charging current to the active transmitting coils  1104   1 - 1104   n  to enable power transfer to a receiving device  1106  in accordance with the charging configuration. The multi-coil wireless charging system  1100  may continue charging until at block  1806 , the multi-coil wireless charging system  1100  detects a step change in a measured power draw. In one example, the measured power draw may be represented by the magnitude of the charging current. In another example, the measured power draw may be represented by a tank voltage. At block  1808 , the multi-coil wireless charging system  1100  may provide a measurement slot  1708  during which the charging current is reduced to quiescent levels. The multi-coil wireless charging system  1100  may measure the quiescent power draw during the measurement slot  1708 . At block  1810 , the multi-coil wireless charging system  1100  may compare the measured quiescent power draw to the Measured Threshold  1716 . The relationship between the measured quiescent power draw and the Measured Threshold  1716  may indicate whether the step change in the power draw is the result of removal of the receiving device  1106 . When the multi-coil wireless charging system  1100  determines at block  1812  that the step change relates to a device removal event, then at block  1814 , the multi-coil wireless charging system  1100  may terminate the charging current. If the multi-coil wireless charging system  1100  determines at block  1812  that the step change does not relate a device removal event, then the process may continue at block  1806 . 
       FIG. 19  is a flowchart  1900  that illustrates a second example of a procedure for device removal detection based on measured quiescent power draw. The procedure may be performed at a multi-coil wireless charging system  1100 . At block  1902 , the multi-coil wireless charging system  1100  may detect the presence of a receiving device  1106  that has been placed on or near the charging surface  1102 . During an initial configuration interval period  1702 , the multi-coil wireless charging system  1100  may interrogate and/or negotiate with the receiving device  1106  to generate a charging configuration. 
     The power transfer period  1704  begins, during which the multi-coil wireless charging system  1100  may provide a charging current to one or more active transmitting coils  1104   1 - 1104   n  configured to wirelessly transfer power to a receiving device  1106  in accordance with the charging configuration. The multi-coil wireless charging system  1100  may continue charging until at block  1904 , the multi-coil wireless charging system  1100  detects a step change in a measured power draw. In one example, the measured power draw may be represented by the magnitude of the charging current. In another example, the measured power draw may be represented by a tank voltage. At block  1906 , the multi-coil wireless charging system  1100  may provide a measurement slot  1708  during which the charging current is reduced to quiescent levels. The multi-coil wireless charging system  1100  may measure the quiescent power draw during the measurement slot  1708 . At block  1908 , the multi-coil wireless charging system  1100  may compare the measured quiescent power draw to an LUT Threshold  1718 . The LUT Threshold  1718  may be premeasured or precalculated based on a quiescent (empty-coil) power draw. The premeasured or precalculated power draw may be maintained in a lookup table stored in non-volatile memory, such as flash memory. The relationship between the measured quiescent power draw to the LUT Threshold  1718  may indicate whether the step change in the power draw is the result of removal of the receiving device  1106 . When the multi-coil wireless charging system  1100  determines at block  1910  that the step change relates to a device removal event, then at block  1912 , the multi-coil wireless charging system  1100  may terminate the charging current. If the multi-coil wireless charging system  1100  determines at block  1910  that the step change does not relate a device removal event, then the process may continue at block  1904 . 
       FIG. 20  is a graph  2000  that illustrates the use of a measurement slot to perform a ping procedure that can determine whether the receiving device  1106  remains on or near the charging surface  1102 . The ping procedure may include an active and/or passive ping. The ping procedure may include an analog and/or digital ping. An initial configuration interval period  2002  may be provided after a device or object has been detected on or near the charging surface  1102 . A ping procedure may be conducted within the initial configuration interval period  2002  to determine whether the detected object is a chargeable object and to determine a charging configuration suitable for a chargeable object. 
     The curve  2010  represents power transfer from the multi-coil wireless charging system  1100  to the receiving device  1106 . During a power transfer period  2004 , an event  2006  may be detected, where the level of power transfer exhibits a step drop. The multi-coil wireless charging system  1100  may initiate a measurement slot  2008  in order to establish or confirm a device removal event. During the measurement slot  2008 , the multi-coil wireless charging system  1100  may terminate the charging current to permit a ping procedure to be used to determine whether the receiving device  1106  has been removed. The multi-coil wireless charging system  1100  may discontinue the charging current after determining that the receiving device  1106  has been removed. The multi-coil wireless charging system  1100  may continue power transfer at a lower level  2012  after determining that the receiving device  1106  has not been removed. 
       FIG. 21  is a flowchart  2100  that illustrates an example of a method for device removal detection based on a ping procedure performed during a measurement slot. The method may be performed at a multi-coil wireless charging system  1100 . At block  2102 , the multi-coil wireless charging system  1100  may detect the presence of a receiving device  1106  that has been placed on or near the charging surface  1102 . During an initial configuration interval period  2002 , the multi-coil wireless charging system  1100  may interrogate and/or negotiate with the receiving device  1106  to generate a charging configuration. 
     The power transfer period  2004  begins, during which the multi-coil wireless charging system  1100  may provide a charging current to one or more active transmitting coils  1104   1 - 1104   n  configured to wirelessly transfer power to a receiving device  1106  in accordance with the charging configuration. The multi-coil wireless charging system  1100  may continue charging until at block  2104 , the multi-coil wireless charging system  1100  detects a step change in a measured power draw, current or tank voltage. At block  2106 , the multi-coil wireless charging system  1100  may provide a measurement slot  2008  during which one or more ping procedures may be executed to determine whether the step change is the result of removal of the receiving device  1106 . When the multi-coil wireless charging system  1100  determines at block  2108  that the step change relates to a device removal event, then at block  2110 , the multi-coil wireless charging system  1100  may terminate the charging current. If the multi-coil wireless charging system  1100  determines at block  2108  that the step change does not relate a device removal event, then the process may continue at block  2104 . 
     Device Removal Detection Using Sensors 
     According to certain aspects of this disclosure, presence, position and/or orientation of a receiving device may be determined using a location sensing technique that involves, for example, detecting differences or changes in capacitance, resistance, inductance, touch, pressure, temperature, load, strain, and/or another appropriate type of sensing. Location sensing may be employed to determine presence or location of an object or device to be charged. Location sensing may also be employed to detect removal of a receiving device during power transfer from a charging surface. 
       FIG. 22  illustrates a first example of a charging surface  2200  of a wireless charger that includes one or more sensors  2202  that can detect removal of a receiving device during power transfer from the charging surface  2200 . In this example, the sensors  2202  may include capacitive, inductive, or hall effect sense elements configured to detect the presence of a device. In some implementations, the sense elements may border the charging coils (LP1-LP18) provided in the charging surface  2200 . In some implementations, the sense elements may border individual charging coils or groups of charging coils. In certain implementations, charging zones may be identified on the charging surface  2200 , and the sense elements may define or monitor the outer limits of each charging zone. 
     The sensors  2202  may also be used to detect changes indicative of removal of a receiving device from the charging surface  2200 . In some implementations, the sensors  2202  may support or enhance removal detection techniques based on measurements of charging current, tank voltages and/or power draw. The use of sensors  2202  may improve reliability, efficiency and can reduce power consumption and processor loading. 
       FIG. 23  illustrates a second example of a charging surface  2300  of a wireless charger that includes one or more sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  that can be used for detecting device removal. The sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  can measure deformation, loading and/or weight attributable to a device or object placed on or near the charging surface  2300 . The sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  may be configured to measure deformation as mechanical strain which may quantify the displacement between two points on a surface. In one example, sensors  2312   1 - 2312   n  placed between transmitter coils  2304   1 - 2304   n  and a circuit board  2302  may provide measurements that correspond to the combined weight of the transmitter coils  2304   1 - 2304   n  and the device or object placed on or near the charging surface  2300 . The weight of the device or object may be calculated from the combined weight, or a change in the combined weight may be used to indicate placement or removal of the device or object. In another example, sensors  2314   1 - 2314   n  placed on the exterior surface of the charging surface  2300  and above the transmitter coils  2304   1 - 2304   n  may provide measurements that correspond to the deformation of the exterior surface caused by the weight and shape of the object placed on or near the charging surface  2300 . 
     The sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  may be used to detect changes indicative of removal of a receiving device  2306  from the charging surface  2300 . In some implementations, the sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  may support or enhance removal detection techniques based on measurements of charging current, tank voltages and/or power draw. The use of sensors  2312   1 - 2312   n  and/or  2314   1 - 2314   n  may improve reliability, efficiency and can reduce power consumption and processor loading. 
       FIG. 24  illustrates a third example of a charging surface  2400  of a wireless charger that includes one or more sensors  2412   1 - 2412   n  used for detecting device removal. The sensors  2412   1 - 2412   n  can measure small changes in movement or vibration caused when a receiving device  2406  or other object is picked up or otherwise removed from the charging surface  2400 . In one example, sensors  2412   1 - 2412   n  are placed between transmitter coils  2404   1 - 2404   n  and a circuit board  2402 . 
     In some implementations, the sensors  2412   1 - 2412   n  may support or enhance removal detection techniques based on measurements of charging current, tank voltages and/or power draw. The use of sensors  2412   1 - 2412   n  may improve reliability, efficiency and can reduce power consumption and processor loading. 
       FIG. 25  illustrates a fourth example of a charging surface  2500  of a wireless charger that includes one or more devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4  that may be used for detecting device removal. The devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4  may include infrared and/or ultrasonic transmitting and sensing devices located co-planar with an exterior surface of the charging surface  2500 . In the illustrated example, the transmitting devices  2504   1 - 2504   4  direct infrared or ultrasonic beams to set of sensing devices  2506   1 - 2506   4 . One or more beams may be interrupted when a receiving device  2502  is placed on the charging surface  2500  between corresponding pairs of the devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4 . Removal of the receiving device  2502  enables one or more of the sensing devices  2506   1 - 2506   4  to detect a transmitted beam. 
     In some implementations, the devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4  may support or enhance removal detection techniques based on measurements of charging current, tank voltages and/or power draw. The use of the devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4  may improve reliability, efficiency and can reduce power consumption and processor loading. In some implementations, increased numbers of transmitting and/or sensing devices  2504   1 - 2504   4  and/or  2506   1 - 2506   4  can provide improved resolution of device location that may be expressed in X and Y coordinates. 
       FIG. 26  illustrates a fifth example of a charging surface  2600  of a wireless charger that includes one or more sensing devices  2604   1 - 2604   5  that may be used for detecting device removal. The sensing devices  2604   1 - 2604   5  may include infrared and/or ultrasonic combined transmitters and sensors. The sensing devices  2604   1 - 2604   5  may be located co-planar with an exterior surface of the charging surface  2600 . In the illustrated example, sensing devices  2604   1 - 2604   5  transmit an infrared or ultrasonic beam and are configured to sense characteristics of reflections of the beams. One or more beams may be reflected by a receiving device  2602  placed on the charging surface  2600 . The sensing devices  2604   1 - 2604   5  may detect phase changes, angles of reflection and other characteristics of the reflected beams, thereby permitting detection of the receiving device  2602 . Removal of the receiving device  2602  eliminates the reflected beams or modifies the characteristics of the reflected beams. 
     In some implementations, the sensing devices  2604   1 - 2604   5  may support or enhance removal detection techniques based on measurements of charging current, tank voltages and/or power draw. The use of the sensing devices  2604   1 - 2604   5  may improve reliability, efficiency and can reduce power consumption and processor loading. In some implementations, increased numbers of sensing devices  2604   1 - 2604   5  can provide improved resolution of device location that may be expressed in X and Y coordinates. In some implementations, the sensing devices  2604   1 - 2604   5  may detect distance between the sensing devices  2604   1 - 2604   5  and the receiving device  2602 , enabling two sensors to determine precise locations of the receiving device  2602 . 
     Example of a Processing Circuit 
       FIG. 27  illustrates an example of a hardware implementation for an apparatus  2700  that may be incorporated in a charging device or in a receiving device that enables a battery to be wirelessly charged. In some examples, the apparatus  2700  may perform one or more functions disclosed herein. In accordance with various aspects of the disclosure, an element, or any portion of an element, or any combination of elements as disclosed herein may be implemented using a processing circuit  2702 . The processing circuit  2702  may include one or more processors  2704  that are controlled by some combination of hardware and software modules. Examples of processors  2704  include microprocessors, microcontrollers, digital signal processors (DSPs), SoCs, ASICs, field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, sequencers, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. The one or more processors  2704  may include specialized processors that perform specific functions, and that may be configured, augmented or controlled by one of the software modules  2716 . The one or more processors  2704  may be configured through a combination of software modules  2716  loaded during initialization, and further configured by loading or unloading one or more software modules  2716  during operation. 
     In the illustrated example, the processing circuit  2702  may be implemented with a bus architecture, represented generally by the bus  2710 . The bus  2710  may include any number of interconnecting buses and bridges depending on the specific application of the processing circuit  2702  and the overall design constraints. The bus  2710  links together various circuits including the one or more processors  2704 , and storage  2706 . Storage  2706  may include memory devices and mass storage devices, and may be referred to herein as computer-readable media and/or processor-readable media. The storage  2706  may include transitory storage media and/or non-transitory storage media. 
     The bus  2710  may also link various other circuits such as timing sources, timers, peripherals, voltage regulators, and power management circuits. A bus interface  2708  may provide an interface between the bus  2710  and one or more transceivers  2712 . In one example, a transceiver  2712  may be provided to enable the apparatus  2700  to communicate with a charging or receiving device in accordance with a standards-defined protocol. Depending upon the nature of the apparatus  2700 , a user interface  2718  (e.g., keypad, display, speaker, microphone, joystick) may also be provided, and may be communicatively coupled to the bus  2710  directly or through the bus interface  2708 . 
     A processor  2704  may be responsible for managing the bus  2710  and for general processing that may include the execution of software stored in a computer-readable medium that may include the storage  2706 . In this respect, the processing circuit  2702 , including the processor  2704 , may be used to implement any of the methods, functions and techniques disclosed herein. The storage  2706  may be used for storing data that is manipulated by the processor  2704  when executing software, and the software may be configured to implement any one of the methods disclosed herein. 
     One or more processors  2704  in the processing circuit  2702  may execute software. 
     Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, algorithms, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside in computer-readable form in the storage  2706  or in an external computer-readable medium. The external computer-readable medium and/or storage  2706  may include a non-transitory computer-readable medium. A non-transitory computer-readable medium includes, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., a compact disc (CD) or a digital versatile disc (DVD)), a smart card, a flash memory device (e.g., a “flash drive,” a card, a stick, or a key drive), RAM, ROM, a programmable read-only memory (PROM), an erasable PROM (EPROM) including EEPROM, a register, a removable disk, and any other suitable medium for storing software and/or instructions that may be accessed and read by a computer. The computer-readable medium and/or storage  2706  may also include, by way of example, a carrier wave, a transmission line, and any other suitable medium for transmitting software and/or instructions that may be accessed and read by a computer. Computer-readable medium and/or the storage  2706  may reside in the processing circuit  2702 , in the processor  2704 , external to the processing circuit  2702 , or be distributed across multiple entities including the processing circuit  2702 . The computer-readable medium and/or storage  2706  may be embodied in a computer program product. By way of example, a computer program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     The storage  2706  may maintain and/or organize software in loadable code segments, modules, applications, programs, etc., which may be referred to herein as software modules  2716 . Each of the software modules  2716  may include instructions and data that, when installed or loaded on the processing circuit  2702  and executed by the one or more processors  2704 , contribute to a run-time image  2714  that controls the operation of the one or more processors  2704 . When executed, certain instructions may cause the processing circuit  2702  to perform functions in accordance with certain methods, algorithms and processes described herein. 
     Some of the software modules  2716  may be loaded during initialization of the processing circuit  2702 , and these software modules  2716  may configure the processing circuit  2702  to enable performance of the various functions disclosed herein. For example, some software modules  2716  may configure internal devices and/or logic circuits  2722  of the processor  2704 , and may manage access to external devices such as a transceiver  2712 , the bus interface  2708 , the user interface  2718 , timers, mathematical coprocessors, and so on. The software modules  2716  may include a control program and/or an operating system that interacts with interrupt handlers and device drivers, and that controls access to various resources provided by the processing circuit  2702 . The resources may include memory, processing time, access to a transceiver  2712 , the user interface  2718 , and so on. 
     One or more processors  2704  of the processing circuit  2702  may be multifunctional, whereby some of the software modules  2716  are loaded and configured to perform different functions or different instances of the same function. The one or more processors  2704  may additionally be adapted to manage background tasks initiated in response to inputs from the user interface  2718 , the transceiver  2712 , and device drivers, for example. To support the performance of multiple functions, the one or more processors  2704  may be configured to provide a multitasking environment, whereby each of a plurality of functions is implemented as a set of tasks serviced by the one or more processors  2704  as needed or desired. In one example, the multitasking environment may be implemented using a timesharing program  2720  that passes control of a processor  2704  between different tasks, whereby each task returns control of the one or more processors  2704  to the timesharing program  2720  upon completion of any outstanding operations and/or in response to an input such as an interrupt. When a task has control of the one or more processors  2704 , the processing circuit is effectively specialized for the purposes addressed by the function associated with the controlling task. The timesharing program  2720  may include an operating system, a main loop that transfers control on a round-robin basis, a function that allocates control of the one or more processors  2704  in accordance with a prioritization of the functions, and/or an interrupt driven main loop that responds to external events by providing control of the one or more processors  2704  to a handling function. 
     In one implementation, the apparatus  2700  includes or operates as a wireless charging device that has a battery charging power source coupled to a charging circuit, a plurality of charging cells and a controller, which may be included in one or more processors  2704 . The plurality of charging cells may be configured to provide a charging surface. At least one coil may be configured to direct an electromagnetic field through a charge transfer area of each charging cell. The controller may be configured to cause the charging circuit to provide a charging current to a resonant circuit when a receiving device is placed on the charging surface, provide a measurement slot by decreasing or terminating the charging current for a period of time, and determine whether the receiving device has been removed from the charging surface based on measurement of a characteristic of the resonant circuit during the measurement slot, wherein the characteristic of the resonant circuit is representative of electromagnetic coupling between a transmitting coil in the resonant circuit and a receiving coil in the receiving device. 
     In certain implementations, the resonant circuit includes a transmitting coil. The controller may be further configured to detect a change or rate of change in voltage or current level associated with the resonant circuit, and provide the measurement slot responsive to detecting the change or rate of change in the voltage or current level. 
     The characteristic of the resonant circuit may be indicative of coupling between the transmitting coil and a receiving coil in the receiving device. In one example, the characteristic of the resonant circuit is a Q-factor. The controller may be further configured to measure a rate of decay of energy stored in the resonant circuit, and determine that the receiving device has been removed from the charging surface when the rate of decay of the energy stored in the resonant circuit corresponds to a rate of decay measured before the receiving device is placed on the charging surface. 
     In some implementations, the apparatus  2700  has one or more sensors located proximate to an exterior surface of the charging device. The controller may be further configured to receive measurements from the one or more sensors, and measure the voltage or current level associated with the resonant circuit when one of the measurements indicates physical removal of the receiving device. The sensors may include a strain measuring sensor, an accelerometer, an infrared or ultrasonic sensing element and/or a hall-effect device. 
     In some implementations, the storage  2706  maintains instructions and information where the instructions are configured to cause the one or more processors  2704  to provide a charging current to a resonant circuit when a receiving device is placed on the charging surface, determine that a change in voltage or current level associated with the resonant circuit indicates a potential removal of the receiving device from the charging surface, provide a measurement slot by decreasing or terminating the charging current for a period of time, and determine whether the receiving device has been removed from the charging surface based on measurement of a characteristic of the resonant circuit during the measurement slot. In one example, the change in the voltage or the current level includes a step change in the voltage or the current level. A low-pass filter may be used to filter short-duration or low-magnitude step changes in a signal representing the voltage or the current level. 
     In certain implementations, the resonant circuit includes a transmitting coil in the charging surface. The characteristic of the resonant circuit may be indicative of coupling between the transmitting coil and a receiving coil in the receiving device. The instructions may be configured to cause the one or more processors  2704  to determine that the receiving device has been removed from the charging surface when a voltage measured at a terminal of the transmitting coil exceeds a threshold voltage level. In some instances, the threshold voltage level may be maintained by a lookup table. In some instances, the threshold voltage level may be determined when the transmitting coil is electromagnetically uncoupled. In some instances, the threshold voltage level may be determined when the receiving device is first placed on the charging surface. 
     In certain implementations, the instructions may be configured to cause the one or more processors  2704  to determine that the receiving device has been removed from the charging surface when a current measured in the resonant circuit has a magnitude that is less than a threshold current level. In some instances, the threshold current level is maintained by a lookup table. In one example, the threshold current level may be determined when no object is electromagnetically coupled with a coil in the resonant circuit. In another example, the threshold current level may be determined when the receiving device is first placed on the charging surface. 
     In some implementations, the instructions may be configured to cause the one or more processors  2704  to determine that the receiving device has been removed from the charging surface based on a rate of decay of energy stored in the resonant circuit. In some implementations, the instructions may be configured to cause the one or more processors  2704  to use a passive ping procedure to determine whether the receiving device has been removed from the charging surface. The passive ping procedure may be performed during the measurement slot and after terminating the charging current. In some implementations, the instructions may be configured to cause the one or more processors  2704  to use a digital ping procedure to determine whether the receiving device has been removed from the charging surface. The digital ping procedure may be performed during the measurement slot and after terminating the charging current. 
     In certain implementations, the instructions may be configured to cause the one or more processors  2704  to monitor and/or receive measurements from one or more sensors in the charging surface. The instructions may be configured to cause the one or more processors  2704  to measure the voltage or current level associated with the resonant circuit after one of the measurements indicates physical removal of the receiving device. The sensors may include a strain measuring sensor, an accelerometer, an infrared or ultrasonic sensing element and/or a hall-effect device. 
       FIG. 28  is a flowchart  2800  illustrating a method for operating a charging device in accordance with certain aspects of this disclosure. The method may be performed by a controller in the charging device. At block  2802 , the controller may provide a charging current to a resonant circuit when a receiving device is placed on a charging surface of the charging device. At block  2804 , the controller may provide a measurement slot by decreasing or terminating the charging current for a period of time. At block  2806 , the controller determines whether the receiving device has been removed from the charging surface based on measurement of a characteristic of the resonant circuit during the measurement slot. The characteristic of the resonant circuit may be representative of electromagnetic coupling between a transmitting coil in the resonant circuit and a receiving coil in the receiving device. If at block  2808 , the controller determines that the receiving device has been removed from the charging surface, then at block  2810  the controller may terminate the charging current and the charging cycle associated with the receiving device. If at block  2808 , the controller determines that the receiving device has not been removed from the charging surface, then the method may continue or resume at block  2804 . The resonant circuit may include a transmitting coil. 
     In certain implementations, the controller may detect a change or rate of change in voltage or current level associated with the resonant circuit, and provide the measurement slot responsive to detecting the change or rate of change in the voltage or current level. 
     In one example, the characteristic of the resonant circuit is indicative of coupling between the transmitting coil and a receiving coil in the receiving device. In some instances, the characteristic of the resonant circuit includes a Q-factor. 
     In certain implementations, the controller may measure a rate of decay of energy stored in the resonant circuit, and determine that the receiving device has been removed from the charging surface when the rate of decay of the energy stored in the resonant circuit corresponds to a rate of decay measured before the receiving device is placed on the charging surface. 
     In certain implementations, the controller may receive measurements from one or more sensors in the charging surface, and may measure the voltage or current level associated with the resonant circuit after one of the measurements indicates physical removal of the receiving device. The sensors may include a strain measuring sensor, an accelerometer, an infrared or ultrasonic sensing element and/or a hall-effect device. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of  35  U.S.C. § 112 , sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”